Dual Connectivity (DC) was introduced for LTE in Rel-12 for inter-frequency heterogeneous deployments, i.e. where macro and pico base stations operate on separate frequencies. By letting the UE transmit and receive data to and from two eNBs at the same time, peak bit rates can be increased by utilizing both frequency layers. By splitting the data higher up in the protocol stack, compared to carrier aggregation, non-ideal backhaul and independent scheduling in the eNBs is supported. Another use case of dual connectivity is control and user plane separation, where user plane data can be offloaded to the pico layer, while maintaining the control plane connection in the macro node. Thus, control and user plane separation may provide a mobility robustness of a macro deployment, while still allowing offloading gains of moving user plane traffic to the pico layer, which is also illustrated in FIG. 1.
Another use case for dual connectivity is uplink, UL, and downlink, DL, separation, where the user uplink is connected for example to pico cell and the user downlink is connected to macro cell. With the support for inter-frequency deployments, Rel-12 provides support for inter frequency UL/DL separation, but an intra-frequency case is not supported in Rel-12 . Nevertheless, some studies show that intra-frequency UL/DL separation could provide much higher gains as compared to inter frequency UL/DL separation.
In heterogeneous networks, the eNBs have different DL output power, e.g., macro eNBs with high output power and pico eNBs with low output power. This imbalance in the transmission power combined with the conventional cell selection mechanism leads to at least two problems:
In LTE, Reference Signal Received Power-based (RSRP-based) cell selection is often used. In this scheme, UEs are associated with the cell from which the strongest DL power is received. As the macro eNB has higher output power than the pico eNB, UEs are more likely to connect to the macro cell. The pico cell size is thus relatively small as compared to the macro cell size, which can result in low UE uptake and small macro offloading by the pico cell. In addition to that, with the RSRP-based cell selection scheme some of the macro connected UEs experience a lower path loss to the pico eNB, and thus are not connected to the best cell from an UL perspective.
To increase offloading of the macro by the pico cells and to improve UL performance, there is a need to increase the size of the pico cells. This can be done with Cell Range Expansion (CRE) based cell selection, where a Cell Selection Offset (CSO) is added to the RSRP of the pico eNB before comparison.
With CRE, a UE may be connected to a pico cell even though the received DL power from the macro cell is stronger. In case of inter-frequency deployment, a large CSO is conceivable for the DL, but in case of intra-frequency deployment, applying a CSO introduces the additional challenge of strong interference. Alternatively to RSRP, Reference Signal Received Quality (RSRQ) can also be used for inter frequency cell selection.
As was discussed above, in heterogeneous networks the DL cell border and the UL cell border are at different places and hence the cell border cannot be set to optimize both UL and DL simultaneously. UL/DL split provides a means to tackle this issue by using dual connectivity to connect the uplink to one eNB and the downlink to another. In dual connectivity, in LTE terminology, one of these base stations is called Master eNB (MeNB) and the other one is called Secondary eNB (SeNB). The serving macro node or the high power node is usually the MeNB and the serving low power node is usually the SeNB, and with UL/DL separation it generally is beneficial to transmit the user data in downlink from the high power node and in uplink to the low power node, which usually is closer to the UE meaning less path loss.
The UL/DL separation can be done on several protocol levels, but since for 3GPP Release 12 the packet data convergence protocol (PDCP) level split architecture was approved for dual connectivity, the case being focused is the UL/DL separation based on PDCP split. The working principle of PDCP level split can be seen in FIG. 2a. 
If a UE capable of UL/DL separation has activated the UL/DL separation between two cells, it means that the uplink user application data is transferred via physical uplink shared channel (PUSCH) of one of those two cells, which will now be called the “uplink cell”, usually the SeNB cell, while the downlink user application data is transferred via physical downlink shared channel (PDSCH) of the other cell, which now will be called the “downlink cell”, usually the MeNB cell. Therefore, the user data will not be transmitted in the uplink of the downlink cell, and similarly the user data will not be transmitted in the downlink of the uplink cell. However, in the PDCP split architecture, the radio link control (RLC) signaling is bi-directional in both of these two cells which means that there is RLC related UL transmissions to both cells on PUSCH and PDSCH respectively.
In a company internal simulation of UL/DL separation the technology potential of UL/DL separation was compared with a reference case. In the reference case, the CSO was set such that DL performance was optimized. The figures resulting from the simulation then showed the gain potential by applying UL/DL separation to the reference case, i.e. by moving macro UEs' uplink to the pico. In a simulated inter-frequency scenario, the cell edge UL user throughput gains were 31% and 38% for low and medium load respectively. While for the intra-frequency scenarios, which are illustrated in FIG. 2b , the cell edge UL user throughput gains were 205% and 59% in low and medium load, respectively. The main sources of the gains seem to be an increased throughput due to a better uplink connection to a closer low power node and offloading traffic from a highly loaded macro cell to small cells with low load levels.
For Rel-12 , intra frequency UL/DL separation was down prioritized in RAN2 based on open issues mainly regarding the physical layer realization, but also regarding the performance evaluations, which did not consider existing features, like e.g. ABS. What may also have impacted the decision was a strong focus on scenario #2 (inter frequency) for Rel-12 . Nevertheless, even when down prioritized during the study item phase, we note that a form of UL/DL separation for inter frequency can be achieved with the split bearer by scheduling only UL via SCG and DL via MCG. However, the most gain potential for UL/DL separation is in intra frequency deployments, and thus support for UL/DL separation in intra frequency deployments is a possible improvement for future versions of the standard.
The Rel-12 user plane protocol architecture supports three different types of bearers, as shown in FIG. 3. The three bearer types are:                A bearer only served by MeNB, referred to as Master Cell Group (MCG) Data Radio Bearer (DRB), i.e. a DRB for which resources are provided by the Master Cell Group. This is depicted with a dashed outline in FIG. 3.        A bearer only served by SeNB, referred to as Secondary Cell Group (SCG DRB), i.e. a DRB for which resources are provided by the Secondary Cell Group. This is depicted with a dash-dot outline in FIG. 3.        A bearer served by MeNB and SeNB, referred to as split DRB. This is depicted with a dotted outline in FIG. 3.        
The UL/DL separation described here is applied to the split bearer. This means that the UE has two RLC connections associated with the split bearer. Furthermore, there are two Medium Access Control (MAC) entities in the UE side in dual connectivity operation: UE side MAC entity is configured per Cell Group, i.e. one MAC for MCG and the other MAC for SCG (solid and dash-double-dot outlines in FIG. 3)
Radio Link Control (RLC) Protocol
The tasks of RLC include segmentation and concatenation, handling retransmission, the detection of duplicates and in-sequence delivery to higher layers. Essentially, the main responsibility of RLC is to transfer user data and signaling between the upper layers and the medium access control (MAC) layer. The RLC provides services for PDCP in the form of radio bearers. The data flows to and from the MAC layer are called logical channels. There is one RLC entity per radio bearer configured for a terminal. Therefore, in the UL/DL separation architecture shown in FIG. 2a , there are two RLC entities per UL/DL separated terminal, one entity for each cell connection. An RLC entity is configured in one of the three possible data transfer modes, which are Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). Only RLC AM is supported for the split bearer.
The RLC AM provides bidirectional data service, meaning that one RLC AM entity is able to both receive and transmit. This makes a feedback channel possible, and therefore enabling retransmissions, which is the most significant feature of a RLC AM.
An UL/DL separated terminal can use RLC AM in the downlink cell, which means that there will also be uplink transmissions in the downlink cell. In an RLC connection the transmitter will first transmit an RLC Packet Data Unit (PDU) to the receiver. The transmitter side will store this PDU in its buffer until it is acknowledged. The transmitter continues with further RLC PDU transmissions, until for example a polling bit is included in the transmission requesting an RLC Status Report (SR) from the receiver side. There are other triggers for the RLC SR as well, for example a fixed timer or a detection of a missing PDU. Then the receiver side transmits an RLC SR PDU towards the transmitter side. In the RLC SR the missing RLC data PDUs are indicated as well as whether it is ready to receive the next RLC PDU. After this the transmitter performs retransmissions of the missing PDUs and continues transmitting new PDUs, which are again ACKed/NACKed by the RLC SR from the receiver. See FIG. 4 for the RLC AM working principle.
The two RLCs of an UL/DL separated terminal are independent, transmitting according to the grants given by their respective cell schedulers. Therefore the two uplink transmissions may happen simultaneously, an RLC SR in the downlink cell and an RLC data PDU in the uplink cell. In intra-frequency DC this is problematic as the transmissions interfere with each other. In addition to RLC SRs, with Rel-12 architecture also RRC control signaling may be transmitted in uplink in the downlink cell causing the same problem as the uplink cell RLC data PDUs.
Instant Uplink Access
Instant Uplink Access (IUA) is a form of prescheduling to allow uplink transmission of data without SR. The IUA solution is based on the Semi-Persistent Scheduling (SPS) framework introducing a new UE condition, namely: “Do not transmit using the grant unless there is data in buffer”. The IUA is a company internal concept. The IUA concept is briefly described here as it is used as part of the solution described herein.
It can be said that, to improve efficiency, IUA introduces two “IUA phases” in the CONNECTED state as depicted in FIG. 5a. 
The first phase starts when the eNB grants the UE with an IUA grant. In this phase the UE has the ability of fast uplink access, but is operating in a low power consumption mode, e.g. DRX, and have poor link adaptation due to minimal communication between the UE and eNB. This phase is referred to as the “inactive” phase in FIG. 5a. 
When the UE gets data to send, it transmits a Buffer Status Report (BSR) and whatever data that fits into the IUA grant. Having received the BSR, the eNB now has the information of UL data in the UE and can start scheduling the UE with dedicated grants. The UE enters the active phase which is similar to the case when an LTE Release-8 UE is granted resources, thus having good throughput and good link adaptation.
In more detail the operation of IUA is depicted in FIG. 6 and listed here:                1 . The UE receives an IUA grant.                    a. The grant is Identified by an IUA C-RNTI                        2 . The UE acknowledges the IUA grant                    a. Using padding PDU if no data is available                            Then the eNB knows IUA grant is received and the eNB can adjust the link adaptation                                                3 . After being granted IUA resources, no padding is sent if there is no data in the buffer        4 . New data is created in the UE and put in the send buffer                    a. A BSR is triggered                        5 . UE transmits the BSR and data using the IUA grant        6 . eNB sends an ACK on PHICH                    a. and a dedicated grant if motivated by the BSR                        7 . The UE transmits data using the dedicated grant        8 . The IUA grant remains valid after the dedicated grants                    a. And can thus be used for subsequent data                        
In LTE all transmitted data is scheduled beforehand both in downlink and uplink and the scheduling is done by the scheduler in the serving base station. Generally the two schedulers in the two serving base stations of the UL/DL separated UE are independent of each other meaning that they schedule the transmissions in their respective cells without taking into account the scheduled transmissions in the other cell.
In the case where the UL/DL separated UE has user data to transmit in uplink (which will be transmitted through the uplink cell) as well as RRC or RLC status report to transmit at the same time in uplink in the downlink cell, the schedulers in these two cells, being independent of each other, can schedule this user to transmit the aforementioned uplink transmissions at least partly within the same radio resource. This means that this user needs to transmit simultaneously to the two base stations with same frequency. In principle this is possible if the UE has dual Tx, that is it has two transmitters. However, since the UE has omnidirectional antennas, these transmissions are likely to cause heavy interference to each other. The uplink RLC status report transmission is perceived as interference in the base station of the uplink cell when receiving the user data, and the user data transmission is perceived as interference in the base station of the downlink cell when receiving the RLC signaling. This interference disturbs the reception of the wanted transmissions and may even cause errors in the reception. The situation is illustrated in FIG. 6.